1. Field of the Invention
The present invention relates to a zoom lens and an imaging apparatus using the same.
2. Description of the Related Art
In recent years, digital cameras that form an image on an imaging surface of an imaging device such as a CCD or C-MOS sensor and electrically record the image have become popular, replacing cameras of a type that expose an image of an object on a film. Users of digital cameras of a popular type desire to easily enjoy photographing at various locations without any restriction of time and place. Therefore, compact-type digital cameras are now popular choices. A use of a small-sized imaging device allows the size of the digital camera to be reduced as compared to that of a camera using a film.
In the filed of such a digital camera, achievement of sufficient miniaturization, wide angle of view, and high zoom ratio is required. As a zoom lens advantageous in ensuring sufficient miniaturization and wide angle of view, a negative lead type zoom lens system in which a lens group having a negative refracting power is arranged nearest the side at which an object is located is disclosed in, e.g., Patent Document 1 (JP-A-2005-321561) and Patent Document 2 (JP-A-2004-318110).
However, in a zoom lens disclosed in Patent Document 1, the total number of lens components included in a first lens group is large, which is disadvantageous in the size reduction of the zoom lens at the time when it is retreated in a collapsed manner in a camera body.
Meanwhile, in a zoom lens disclosed in Patent Document 2, only one lens component is included in a first lens group, which is disadvantageous in achieving the wide angle of view and high zoom ratio in view of influence on off-axis aberration.
Further, in the zoom lenses disclosed in the above Patent Documents, the number of bonding surfaces is only one although a second lens group has three or more lenses, which is disadvantageous in achieving the high zoom ratio in view of influence on axial chromatic aberration and chromatic aberration of magnification. In addition, positioning of respective lens components included in the second lens group is made through a frame, making it easy to cause decentering among the lenses, which may result in occurrence of aberration due to manufacturing error.
The present invention has been made in view of the above problems and an object thereof is to provide a zoom lens advantageous in achieving wide angle of view, high zoom ratio, and sufficient miniaturization while maintaining optical performance thereof and an imaging apparatus using the zoom lens.
In view of the above problems, according to a first aspect of the present invention, there is provided a zoom lens including in order from an object side: a first lens group with a negative refracting power; a second lens group with a positive refracting power; a third lens group with a positive refracting power; and a fourth lens group with a refracting curved surface, wherein upon zooming from a wide-angle end to telephoto end, at least the first lens group, second lens group, and third lens group move, and a spacing between the respective lens groups are changed.
The first lens group includes, in order from the object side to an image side, a negative lens component with a negative refracting power and a positive lens component with a positive refracting power which are arranged on an optical axis with a predetermined spacing therebetween, and the total number of the lens components in the first lens group is two.
The second lens group includes at least three lenses including at least one negative lens and at least one positive lens and includes a plurality of cemented surfaces, at least two of which are obtained by cementing two face-to-face refracting surfaces of two lenses each having different curvature on the object side surface and image side surface.
The lens component is a lens body having only two refracting surfaces that contact the air on the optical axis: one is the object-side surface and the other is the image-side surface.
In the case where the zoom lens has a focusing function, the zoom lens is assumed to be focused on the farthest point.
As described above, the configuration in which the first lens group has a negative refracting power is advantageous in a reduction of the number of lens groups constituting the zoom lens and widening of the angle of view. The configuration in which the second lens group has a positive refracting power can impart a zoom function to this second lens group. The configuration in which the third lens group has a positive refracting power is advantageous in adjustment of the exit pupil position at the wide-angle end and telephoto end and in correction of field curvature. The configuration in which the fourth lens group having a refracting curved surface is arranged on the image side of the third lens group is advantageous in a reduction in field curvature at the wide-angle end. Further, the configuration in which the third lens group moves upon focusing is advantageous in a reduction in field curvature on a short-distance object point.
The configuration in which the first lens group includes, in order from the object side to the image side, a negative lens component with a negative refracting power and a positive lens component with a positive refracting power makes the principal point of the first lens group closer to the object side to easily reduce the diameter size of the first lens group or the entire length of the zoom lens. Further, this configuration is advantageous in a reduction in chromatic aberration of the first lens group and reduction in on-axis aberration at the telephoto end, in spite of a small number of lens components.
When the second lens group is configured as described above, a reduction in on-axis aberration and in chromatic aberration of magnification of the second lens group are advantageously achieved. In addition, the second lens group has a plurality of cemented surfaces. That is, a plurality of lenses are directly jointed to one another without use of mirror frames, making it easy to reduce influence on aberration due to relative decentering between lenses. Thus, wide angle of view and high zoom ratio are advantageously achieved.
Preferably, the abovementioned lens satisfies the following requirements.
It is preferable that the zoom lens have an aperture stop that moves in unison with the second lens group upon zooming from the wide-angler end to telephoto end. In this case, the beam diameter in the second lens group can be made small, which is advantageous in a reduction in the size of the zoom lens and reduction in aberration. The aperture stop may be arranged on the object side of the second lens group, on the image side thereof, or in the second lens group. When the aperture stop is arranged on the image side of the second lens group, correction of on-axis aberration is advantageously achieved.
It is preferable that the second lens group satisfy the following conditional expression (1):0.8<DG2/fw<3.0  (1)where DG2 is thickness of the second lens group on the optical axis, and fw is focal length of the entire zoom lens system at the wide-angle end.
The conditional expression (1) represents a preferable thickness of the second lens group on the optical axis. Abiding by the lower limit of the conditional expression (1) advantageously reduces a variation of aberration at the wide-angle end and telephoto end upon zooming and a reduction in spherical aberration and astigmatism. Abiding by the upper limit of the conditional expression (1) advantageously reduces the thickness of the zoom lens.
It is preferable that the second lens group include a plurality of positive lenses, one of which nearest to the object side and one of which nearest to the image side satisfy the following conditional expression (2):0.5<φG2PF/φG2PR<25.0  (2)where φG2PF is refracting power of the most object-side positive lens of the plurality of positive lenses included in the second lens group, and φG2PR is refracting power of the most image-side positive lens thereof.
The conditional expression (2) represents preferable distribution of a refracting power for the positive lens in the second lens group. Abiding by the lower and upper limits of the conditional expression (2) enables setting of the respective refracting powers (inverse numbers of focal lengths) of the positive lens to adequate values, thereby advantageously reducing on-axis aberration, which leads to a high zoom ratio.
When the total number of the positive lenses in the second lens group is set to two, a size reduction of the second lens group and low cost production can be achieved.
It is preferable that the total number of the cemented surfaces in the second lens group satisfy the following conditional expression (3):2≦N2Gcem≦4  (3)where N2Gcem is total number of the cemented surfaces in the second lens group, which is an integer.
The conditional expression (3) represents a preferable total number of the cemented surfaces in the second lens group. Abiding by the lower limit of the conditional expression (3) advantageously corrects on-axis aberration, field curvature, and astigmatism. Abiding by the upper limit of the conditional expression (3) reduces the total number of lenses in the second lens group, advantageously reducing the thickness of the zoom lens at the time when it is retreated in a collapsed manner in a camera body.
Further, it is more preferable that the lens component including the cemented surface have a positive lens and a negative lens, which is advantageous in correction of various aberrations.
It is preferable that the first lens group satisfy the following conditional expression (4):0.10<DL12/DG1<0.80  (4)where DG1 is thickness of the first lens group on the optical axis, and DL12 is distance on the optical axis between the negative lens component and positive lens component in the first lens group.
The conditional expression (4) represents a preferable distance between the first and second lens components in the first lens group. Abiding by the lower limit of the conditional expression (4) makes the principal point of the first lens group closer to the object side while reducing field curvature at the wide-angle end to easily reduce the diameter size of the first lens group, thereby advantageously achieving a wide angle of view. Abiding by the upper limit of the conditional expression (4) advantageously reduces the size of the zoom lens at the time when it is retreated in a collapsed manner in a camera body.
It is preferable that the negative lens component in the first lens group satisfies the following conditional expression (5):−1.0<(r1a+r1b)/(r1a−r1b)<0.98  (5)where r1a is paraxial curvature radius of the object side surface of the negative lens component in the first lens group, and r1b is paraxial curvature radius of the image side surface of the negative lens component in the first lens group.
The conditional expression (5) represents a preferable shape of the negative lens component in the first lens group. Abiding by the lower and upper limits of the conditional expression (5) enables setting of the curvatures of the object side surface and image side surface to adequate values, thereby preventing occurrence of field curvature at the wide-angle end and spherical aberration at the telephoto end.
Further, it is preferable that the total number of the lens components in the second lens group be one, two, or three. With this configuration, it is possible to satisfactorily correct the on-axis aberration while reducing the thickness of the zoom lens at the time when it is retreated in a collapsed manner in a camera body.
Further, it is preferable that the sum of the total number of the lens components in the third lens group and that of the lens components in the fourth lens group be three or less. This configuration is advantageous in the size reduction.
Further, it is preferable that the fourth lens group have a negative refracting power and that the zoom lens be a four-group zoom lens. The configuration in which the zoom lens includes, in order from the object side, a first lens group with a negative refracting power, a second lens group with a positive refracting power, a third lens group with a positive refracting power, and a fourth lens group with a negative refracting power makes the arrangement of signs (+, −) of the refracting power symmetrical, which is advantageous in correction of on-axis aberration and reduction in the size of the zoom lens.
It is preferable that the following conditional expression (6) is satisfied:0.5<DG3G4/DL12<3  (6)where DG3G4 is distance on the optical axis between the third lens group and fourth lens group at the wide-angle end, and DL12 is distance on the optical axis between the negative and positive lens components in the first lens group.
The conditional expression (6) is a conditional expression for satisfactorily achieving symmetry in an optical system in which negative-positive-positive-negative lens groups are arranged in order from the object side so as to satisfactorily correct the entire aberration. In particular, this configuration is effective for off-axis aberration. Abiding by the lower and upper limits of the conditional expression (6) satisfactorily achieves symmetry in the arrangement of the refracting powers in the entire zoom lens, thereby advantageously reducing various aberrations, in particular, off-axis aberration.
It is preferable that the first lens component satisfy the following conditional expression (7):1.87<AVE(ndG1)<2.40  (7)where AVE(ndG1) is average value of the refractive indices of all the lenses in the first lens group.
The conditional expression (7) specifies a preferable average value of the refractive indices of the lenses constituting the first lens group. By abiding by the lower limit of the conditional expression (7) to ensure the refractive index, it is easily possible to ensure the refractive index of the first lens group required to ensure a satisfactory angle of view or zoom ratio while reducing occurrence of aberration at each lens surface. Abiding by the upper limit of the conditional expression (7) advantageously reduces material cost.
According to a second aspect of the present invention, there is provided a zoom lens including in order from the object side: a first lens group with a negative refracting power; a second lens group with a positive refracting power; and a third lens group with a positive refracting power.
Upon zooming from a wide-angle end to telephoto end, at least the first lens group, second lens group, and third lens group move, and a spacing between the respective lens groups are changed.
The second lens group includes at least three lenses including at least one negative lens and at least one positive lens and includes a plurality of cemented surfaces, at least two of which are obtained by cementing two face-to-face refracting surfaces of two lenses each having different curvature on the object side surface and image side surface.
The first lens group satisfies the following conditional expression (7):1.87<AVE(ndG1)<2.40  (7)where AVE(ndG1) is average value of the refractive indices of all the lenses in the first lens group.
As described above, when the conditional expression (7) is satisfied, it is possible to obtain a negative refracting power of the first lens group required to ensure a satisfactory angle of view or zoom ratio while reducing cost of the first lens group and occurrence of aberration in the first lens group.
Thus, it is possible to advantageously reduce the number of lenses constituting the first lens group, ensure a satisfactory angle of view, and reduce the entire lens length.
The function of the second lens group in the second aspect has been described in the first aspect of the present invention.
Further, it is preferable that the zoom lens include a fourth lens group having a refracting curved surface on the image side of the third lens group. This configuration is advantageous in a reduction in field curvature.
In the case where the third lens group moves upon focusing, it is possible to advantageously reduce a variation of field curvature.
Further, it is preferable that the invention according to the second aspect and any of the above configuration requirements be satisfied at the same time.
Further, it is preferable that the fourth lens group remain fixed upon zooming. The configuration in which the fourth lens group remains fixed upon zooming allows significant simplification of the mechanical structure, thereby reducing the thickness of the zoom lens at the time when it is retreated in a collapsed manner in a camera body and cost.
Further, it is preferable that the third lens group move for focusing and that the first, second, and fourth lens groups remain fixed upon the focusing operation. The configuration in which the fourth lens group remains fixed also upon focusing allows significant simplification of the mechanical structure, thereby reducing the thickness of the zoom lens at the time when it is retreated in a collapsed manner in a camera body and cost.
Further, according to another aspect of the present invention, there is provided an imaging apparatus including: a zoom lens; and an imaging device having an imaging surface arranged on the image side of the zoom lens and is adapted to convert an optical image formed on the imaging surface into electric signals, wherein the zoom lens is any of the zoom lenses described above.
In this case, it is preferable that imaging apparatus include an image transformation section that transforms the electric signals containing distortion from the zoom lens into image signals with distortion corrected by image processing.
In an image obtained through a zoom lens, trade-off relationship may be found between the distortion and astigmatism at the wide-angle end. Therefore, recording or display of the image is performed after the astigmatism is optically corrected and barrel distortion likely to occur at the wide-angle end is electrically corrected, leading to a size reduction of the zoom lens and an improvement in quality of an image to be recorded.
Further, it is preferable that the imaging apparatus include an image transformation section that transforms the electric signals including chromatic aberration of magnification from the zoom lens into image signals in which the chromatic aberration of magnification has been corrected by image processing. For example, when the distortion is electrically corrected for each color signal, the chromatic aberration of magnification is accordingly corrected. This allows sharing of occurrence of the chromatic aberration of magnification of the zoom lens itself, which is advantageous in a reduction in material cost and zoom lens size.
It is preferable that a plurality of the above configuration requirements be satisfied at the same time. Further, when following changes are made to the respective conditional expressions, the effect can reliably be demonstrated.
In the conditional expression (1), the lower limit value is preferably 0.9 and more preferably 1.0, and upper limit value is preferably 2.0 and more preferably 1.5.
In the conditional expression (2), the lower limit value is preferably 0.7, and upper limit value is preferably 5.0 and more preferably 1.0.
In the conditional expression (3), the upper limit value is preferably 3, and N2Gcem is preferably 2.
In the conditional expression (4), the lower limit value is preferably 0.2 and more preferably 0.25, and upper limit value is preferably 0.6 and more preferably 0.4.
In the conditional expression (5), the lower limit value is preferably 0.1 and more preferably 0.4, and upper limit value is preferably 0.85 and more preferably 0.75.
In the conditional expression (6), the lower limit value is preferably 1.0 and more preferably 1.2, and upper limit value is preferably 2.7 and more preferably 2.5.
In the conditional expression (7), the lower limit value is preferably 1.90 and more preferably 1.92, and upper limit value is preferably 2.3 and more preferably 2.2.
As described above, according to the present invention, it is possible to provide a zoom lens advantageous in achieving wide angle of view, high zoom ratio, and sufficient miniaturization while maintaining optical performance thereof and an imaging apparatus using the zoom lens.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.